Photovoltaic element and production method therefor

ABSTRACT

A photovoltaic element with a low shadow, a high energy conversion efficiency, a high freedom in dimension and a high reliability with prolonged use is provided. The photovoltaic element comprises a photovoltaic layer having a first semiconductor junction layer for generating an electromotive force, a current collecting electrode provided at the light incident side of the photovoltaic layer, and a bypass diode connected in parallel, wherein the bypass diode is provided under the current collecting electrode as a bypass diode layer having a second semiconductor junction layer other than the first semiconductor junction layer of the photovoltaic layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photovoltaic element with excellentcharacteristics and high reliability and a method for producing thephotovoltaic element, and more specifically to a photovoltaic elementwith excellent characteristics and a reduced loss in energy conversionefficiency due to the formation of a bypass diode under a currentcollecting electrode and a method for producing the photovoltaicelement.

2. Related Background Art

A thin film type solar cell employing amorphous semiconductor isconsidered promising due to its advantages such as its capabilities offorming a large-area solar cell, making the film thickness of asemiconductor thinner and depositing a film on an arbitrary substrate,as compared to a single-crystal or polycrystal type solar cell.

An amorphous silicon type solar cell is formed, for example, by stackingp-, i- and n-type thin amorphous silicon layers on a substrate. Also,for improving the energy conversion efficiency, there is contemplated aso-called double or triple cell structure in which two or more of theabove-mentioned pin junctions are superposed in series. At the lightincident side and the back side of the above-mentioned semiconductor,there are formed a pair of electrodes, namely an upper electrode and alower electrode. In the amorphous silicon type solar cell, because ofthe generally high sheet resistance of the semiconductor itself, thereis required a transparent upper electrode covering the entire area ofthe semiconductor, which is usually composed of a transparent conductivefilm such as of SnO₂ or ITO. Such transparent conductive film functionsalso as an anti-reflective film. On the upper electrode mentioned above,there is provided a current collecting grid electrode which is formedinto a comb-shaped pattern so as not to hinder the entry of light, inother words, the irradiation of light, and a busbar is provided in orderto collect the current from the grid electrode.

As an electric power supply source, a single solar cell (photovoltaicelement) is incapable of supplying a sufficient output voltage. For thisreason, it is necessary to use plural solar cells in parallel or serialconnection. The largest difficulty in utilizing the plural cells(elements) in serial connection as described above relates to thesituation wherein no electric power is generated due to a part of thecells being shadowed from the sunlight, for example, by a building or byaccumulated snow. A serially connected solar cell module can no longergenerate an electric power even though other cells in the module stillgenerate electric power, and the total voltage generated by the normallyfunctioning cells is applied, as a backward voltage, to such a shadowedcell. When such a backward voltage exceeds the tolerable voltage of theelement, destruction thereof may result. In order to prevent such aproblem in the electric power generation or the destruction of theelement, it is necessary to connect, for each of the serially connectedelements, a diode parallel to the element but in a direction opposite tothat of the semiconductor junction of the element. Such a diode isgenerally called a bypass diode.

The application of the bypass diode to the solar cell is, for example,disclosed in Japanese Patent Application Laid-Open No. 5-152596, inwhich a mold-packaged diode is connected in parallel to each solar cell.FIG. 9 is a schematic view showing an example of the solar cell moduleutilizing such bypass diode. FIG. 9 shows a solar cell module 91connected to bypass diodes, solar cells 92, bypass diodes 93, wirings94, wirings 95 for serially connecting the solar cells 92, a glass plate96, an encapsulating resin 97, and a back plastic material 98. As thediode 93 has a thickness of about 3 mm in diameter in the case of ausual axial diode, the encapsulating resin 97 has to be madecorrespondingly thick.

There is proposed a method of incorporating a diode in a semiconductorconstituting a solar cell, as another method of the prior art, becausethe attachment of an independent diode to the solar cell is consideredto increase the thickness of a solar cell module by the thickness of thediode and complicate the manufacturing process with respect to thewiring work. Such a proposal is disclosed, for example, in JapanesePatent Application Laid-Open No. 4-42974, in which a pn junctionfunctioning as a solar cell and a pn junction serving as a bypass diodeare formed on the same substrate in such a manner that they are mutuallyconnected in parallel.

However, with respect to such a conventional photovoltaic element withthe bypass diode, (1) in the photovoltaic element employing an amorphoussemiconductor film formed on the aforementioned substrate, there has notbeen disclosed a method of forming the bypass diode on the samesubstrate, (2) the configuration and the producing method require amasking process, which is complex and lacks freedom in size, and (3) thearea of the bypass diode constitutes a loss in the effective area of thephotovoltaic element; in other words, it is necessary to increase thearea of the solar cell by the area of the bypass diode.

SUMMARY OF THE INVENTION

In consideration of the above-mentioned problems of the conventionaltechnology, an object of the present invention is to provide aphotovoltaic element formed by depositing a photovoltaic element portionand a bypass diode portion on the same substrate, while applying asemiconductor obtained by film formation to the portions, withoutinvolving a complex process, whereby the bypass diode portion does notreduce the effective area of the photovoltaic element with a highfreedom in size, and a producing method therefor.

In order to attain the above-mentioned object, the present inventionprovides a photovoltaic element comprising: a photovoltaic layer havinga first semiconductor junction layer for generating a photoelectromotiveforce, a current collecting electrode provided at the light incidentside of the photovoltaic layer, and a bypass diode connected inparallel, wherein the bypass diode is formed under the currentcollecting electrode as a bypass diode layer having a secondsemiconductor junction layer other than the first semiconductor junctionlayer of the photovoltaic layer.

Also, the present invention provides a method of producing aphotovoltaic element, which comprises a step of forming, on a conductivesubstrate or a substrate with a conductive film formed thereon, aphotovoltaic layer having a first semiconductor junction layer forgenerating a photoelectromotive force in plural positions with apredetermined space, a step of forming a bypass diode layer having asecond semiconductor junction layer with a forward direction of asemiconductor junction opposite to that of the first semiconductorjunction layer, on the substrate between the plural positions of thephotovoltaic layer, and a step of forming a current collecting electrodeso as to be connected to the photovoltaic layer and the bypass diodelayer.

Because the bypass diode layer is formed under the current collectingelectrode, the photovoltaic layer, for example, having a pin or pnsemiconductor junction deposited on a substrate, and the bypass diodelayer can be easily formed by forming a film on the same substrate.Consequently the surface of the element can be planar and there can bedispensed with the step of wiring the bypass diode which is conducted byusing a separate component in the conventional technology. Also themanufacturing process is simplified and is improved in reliability andproduction yield. Furthermore, the bypass diode layer does not sacrificethe effective area of the photovoltaic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an equivalent circuit of the photovoltaicelement according to the present invention;

FIG. 2 is a schematic cross-sectional view showing on example of thesemiconductor layer configuration of the photovoltaic element accordingto the embodiment of the present invention;

FIGS. 3A, 3B and 3C are schematic cross-sectional views showing threedifferent configurations of the photovoltaic element according to thepresent invention;

FIGS. 4A and 4B are respectively a plan view and a cross-sectional viewshowing the entire photovoltaic element shown in FIGS. 3A, 3B and 3C;

FIG. 5 is a schematic cross-sectional view showing the configuration ofa photovoltaic element according to Example 3 of the present invention;

FIGS. 6A and 6B are schematic views showing a method of producing thephotovoltaic element according to Example 3 of the present invention;

FIGS. 7A and 7B are schematic views showing a part of the producingapparatus to be employed in the producing method shown in FIGS. 6A and6B;

FIGS. 8A and 8B are schematic views showing a method of obtaining thephotovoltaic element by division, according to Example 4 of the presentinvention; and

FIG. 9 is a schematic cross-sectional view showing the serialconfiguration of the conventional photovoltaic elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present embodiment, the semiconductor material for forming thephotovoltaic element may be crystalline or amorphous, but is preferablyone capable of forming a thin semiconductor film on the substrate. Suchthin semiconductor films are usually formed by a usual vacuum filmforming process, but similar effects can also be obtained by a liquidphase process. The photovoltaic layer and the bypass diode layer may besimultaneously formed in the above-mentioned film forming process, butthey may also be formed separately with different film formingapparatuses. When the substrate is long, the film formation can beexecuted by a roll-to-roll process. In view of productivity, there ispreferably used a process in which the semiconductor layers aredeposited in succession while the substrate is transported throughplural film forming chambers. Also, there is a leaf-by-leaf process, inother words, a sheet-by-sheet process in which the semiconductor layersare deposited in succession in the course of transportation of thesubstrate. This process is also preferable.

In the present embodiment, the forward direction of each junction of thefirst and second semiconductor junction layers is mutually opposite.More specifically, the bypass diode layer as the second semiconductorjunction layer is formed so as to have a pn or pin junction with aforward direction opposite to that of the photovoltaic layer as thefirst semiconductor junction layer, so as to be provided in parallelwith respect to the photovoltaic layer, and so as to maintain thesurface of the layers at the same level. In order to change thesemiconductor film formed on the substrate, a mask can be used. Morespecifically, in the case of forming the amorphous semiconductor by avacuum process utilizing plasma CVD, there may be positioned a plate forcovering the substrate in a part of the vacuum chamber, for example, inthe width direction of the substrate where the current collectingelectrode is to be formed, in order to prevent film formation in such acovered position.

The photovoltaic element of the present embodiment is produced byforming a photovoltaic layer and the bypass diode layer on the samesubstrate, but the bypass diode layer has to be connected in parallel tothe photovoltaic layer and formed so that the surface of the bypassdiode is the same level as that of the photovoltaic layer. Also theforward directions of semiconductor junctions of the layers have to bemutually opposite to each other. FIG. 1 shows an equivalent circuithaving the connection of the photovoltaic element and the bypass diode.FIG. 1 shows a diode component 11 of the photovoltaic layer, a DC powersource component 12 of the photovoltaic layer, and a bypass diode layer13. The above-described configuration can be obtained by forming onepolar of the photovoltaic layer and one polar of the bypass diode layeron the same substrate, and mutually connecting the other polars with ametal electrode. Specifically, the photovoltaic layer and the bypassdiode layer are both connected to the current collecting electrode asthe metal electrode at the light incident side.

The above-described configuration is shown in FIG. 2. FIG. 2 is aschematic cross-sectional view showing one example of the semiconductorlayer configuration of the photovoltaic element according to the presentembodiment. As shown in FIG. 2, a semiconductor layer 21 is provided ona substrate 22, and linear current collecting electrodes 25 are furtherprovided thereon with an interval therebetween. The side of the currentcollecting electrodes 25 is a light incident side. The semiconductorlayer 21 has a bypass diode layer 23 and a photovoltaic layer 24. Thebypass diode layer 23 and the photovoltaic layer 24 are alternatelyprovided adjacent to each other. The current collecting electrode 25 isformed just on the bypass diode layer 23 so as not to hinder incidentlight. Further, the current collecting electrode 25 is connected to thebypass diode layer 23, and the ends of the current collecting electrode25 are connected to the photovoltaic layer 24. The bypass diode layer 23is composed of a plurality of semiconductor layers 231 and 232 differentin characteristics from each other in this order from thelight-receiving side toward the side of the substrate 22. FIG. 2 showsone example in which the bypass diode layer has a two-layered structureof a p-type layer and an n-type layer. Similar to the bypass diode layer23, the photovoltaic layer 24 is composed of a plurality ofsemiconductor layers 241 and 242 different in characteristics from eachother, and, as one example the photovoltaic layer is composed of ap-type layer and an n-type layer. The semiconductor layers 231 and 241are different in characteristics from each other, in other words, whenone semiconductor layer is a p-type layer, the other semiconductor layeris an n-type layer. Similarly, the semiconductor layers 232 and 242 aredifferent in characteristics from each other.

The photovoltaic layer and the bypass diode layer are laterally formedadjacent to each other on the same substrate and are formed in mutuallyparallel belt-like shapes of the substantially same length, both in theroll-to-roll feeding method or in the leaf-by-leaf feeding method. Byforming these layers in mutually parallel belt-like shapes substantiallyequal in length of at least a long side thereof, and by dividing a largephotovoltaic element along the long side of the belt-like shape, it ispossible to obtain a small photovoltaic element of an arbitrary widthwhich contains a desired number of the belt-like photovoltaic layer andthe belt-like bypass diode layer.

The photovoltaic element of the present embodiment may be produced,except for one shown in FIG. 2, by simultaneously forming at least oneof the semiconductor layers constituting the first semiconductorjunction layer and at least one of those constituting the secondsemiconductor junction layer with the same material. For example, wheneach of the first and second semiconductor junction layers is a pinjunction layer including an intrinsic layer, the intrinsic layer may becontinuously extended over both semiconductor junction layers. In such acase, the intrinsic layer (i-type layer) may be provided in common toboth the first and second semiconductor junction layers, though theorder of deposited layers of the pin junctions is different in the firstand second semiconductor junction layers. Such configuration may beobtained by forming the i-type layers of both junction layers with thesame material at the same time during formation of the first and secondsemiconductor junctions layers with a pin junction. After forming thei-type layer, the surface of a layer formed on the i-type layers on theboth junction layers can be adjusted to the same level.

The first semiconductor junction layer as the photovoltaic layer mayhave a triple cell structure of three stacked semiconductor layers eachhaving a pin or pn junction, and the second semiconductor junction layeras the bypass diode layer may have a single cell structure of a singlesemiconductor layer having a pin or pn junction. Such a configurationallows simplification of the manufacturing process. For example, whenthe first semiconductor junction layer is composed of three stacked pinjunctions, the bypass diode layer can be formed during formation of thep-type layer of the bottom pin junction, the i-type layer of the middlepin junction and the n-type layer of the top pin junction. That is, thethird, fifth and seventh layers among nine layers constituting the firstsemiconductor junction layer are respectively formed simultaneously withthe first, second and third layers constituting the second semiconductorjunction layer using the same materials. In other words, the third layeramong nine layers constituting the first semiconductor junction layer isformed simultaneously with the first layer constituting the secondsemiconductor junction layer using the same material; the fifth layeramong nine layers constituting the first semiconductor junction layer isformed simultaneously with the second layer constituting the secondsemiconductor junction layer using the same material; and the seventhlayer among nine layers constituting the first semiconductor junctionlayer is formed simultaneously with the third layer constituting thesecond semiconductor junction layer using the same material.

First Semiconductor Layer

The semiconductor layers constituting the first semiconductor junctionlayer can be composed of thin semiconductors such as amorphous silicon,microcrystalline silicon or polycrystalline silicon. In case the presentinvention is applied to a pin-type amorphous silicon solar cell, thesemiconductor material constituting the i-type layer can includeso-called Group IV or Group VI alloy type amorphous or microcrystallinesemiconductors such as a-Si:H, a-Si:F, a-Si:H:F, a-SiGe:H, a-SiGe:F,a-SiGe:H:F, a-SiC:H, a-SiC:F or a-SiC:H:F. The semiconductor materialconstituting the p-type or n-type layer can be obtained by doping theabove-mentioned semiconductor material constituting the i-type layerwith a valence electron controlling substance. As the valence electroncontrolling substance for obtaining the p-type semiconductor, there isemployed a compound containing an element of Group III of the periodictable. The element of Group III includes B, Al, Ga and In. Also as thevalence electron controlling substance for obtaining the n-typesemiconductor, there is employed a compound containing an element ofGroup V of the periodic table. The element of Group V includes P, N, Asand Sb.

The amorphous or microcrystalline silicon semiconductor layer can beformed by a known method such as evaporation, sputtering, plasma CVD,microwave plasma CVD, VHFCVD, ECR, thermal CVD or LPCVD. For industrialapplication, there is principally employed RF plasma CVD in which a rawmaterial gas is decomposed by RF plasma and deposited onto thesubstrate. The RF plasma CVD process is associated with drawbacks inthat the decomposition efficiency of the raw material gas is as low asabout 10% and the deposition rate is as low as 1 to 10 Å/sec; microwaveplasma CVD and VHF plasma CVD are contemplated in order to overcomethese drawbacks. As the apparatus for executing the film formationmentioned above, there can be employed a known film-forming apparatus ofbatch or continuous type depending on necessity. The photovoltaicelement of the present invention is applicable also to so-called tandemcells in which two or more semiconductor junctions are stacked in orderto improve the spectral sensitivity or to increase the output voltage.

In order to individually form the photovoltaic layer and the bypassdiode layer, there may be provided a deposition preventing plate (mask)for preventing the film deposition in the vacuum chamber. In this case,the leaf-by-leaf feeding apparatus may be used. Also, the firstsemiconductor junction layer of the photovoltaic layer is formed in theorder of n, i and p, while the second semiconductor junction layer ofthe bypass layer is formed in the order of p, i and n.

Next, the members constituting the solar cell having the photovoltaicelement according to the present embodiment are described. The structureof this solar cell is shown in FIGS. 4A and 4B, but the explanation ofFIGS. 4A and 4B will be described later.

Substrate

Though the substrate is not essential in the present invention, anadvantageous configuration can be obtained by forming thin semiconductorfilms on a substrate with an appropriate shape or size. For example,when the substrate is composed of a metal, the substrate can be used notonly for mechanically supporting the thin films such as thesemiconductor layers and the electrode layer, but also for functioningas an electrode (first electrode). The substrate can be composed of aconductive or insulating material, but when the substrate is composed ofan insulating material, the surface of the substrate is subjected to aconductive treatment to utilize it as an electrode. Such a substrate isrequired to have heat resistance capable of withstanding the heatingtemperature in formation of the semiconductor layers and the electrodelayer. It is also required to be of a continuous long form in the caseof the roll-to-roll film-forming process, and further required to bedimensionally stable without elongation, in order to enable windingunder tension.

Among the substrate materials meeting the above-described requirements,the preferred conductive substrate includes a thin plate composed of ametal such as Fe, Ni, Cr, Al, Mo, Au, Nb, Ta, V, Ti, Pt, Pb or Ti, analloy thereof such as brass or stainless steel, or a composite materialthereof, a carbon sheet or a zinc-plated steel plate. Particularlypreferred is stainless steel because of various features such assatisfactory heat resistance to the heating temperature during filmformation and high mechanical strength suitable for continuous filmformation such as in a roll-to-roll system, for example, even in thecase of a thickness as small as 0.15 mm. Also, among the substratematerials, a preferred insulating substrate includes a heat-resistantresin film or sheet such as polyester, polyethylene, polycarbonate,cellulose acetate, polypropylene, polyvinyl chloride, polyvinylidenechloride, polystyrene, polyamide, polyimide, epoxy resin, a compositematerial of such resin material with glass fibers, carbon fibers orboron fibers, glass or a ceramic material. Particularly preferred is aglass or polyimide substrate.

Upper Electrode

The photovoltaic layer may have an upper electrode at the light incidentside thereof. Such an upper electrode is not essential to the presentinvention, but is provided for reducing the sheet resistance in case thefirst semiconductor junction layer is composed of a material of highresistance such as amorphous silicon. It is not necessary to provide theupper electrode when the sheet resistance is low as in the case ofcrystalline materials including microcrystal. The upper electrode servesto gather the electromotive force generated in the first semiconductorjunction layer and functions as a pair with the first electrode at thesubstrate side. The upper electrode is required to gather the current ina direction parallel to the substrate when utilizing a semiconductormaterial of high sheet resistance such as amorphous silicon, andpreferably has a sheet resistance not exceeding 300 Ω/□. The thicknessof the upper electrode must be designed so as to have a sufficiently lowresistance and satisfactory transparency, and in some cases, to minimizethe light reflection at the wavelength of the light to be transmitted,based on the light interference condition. For example, in order tominimize the reflection of light of 550 nm using ITO as the upperelectrode, there is preferred a thickness of about 700 Å. Also, theabove-mentioned upper electrode (second electrode) is positioned at thelight incident side and preferably has a light transmittance of at least85% in order that the light from the sun or a white fluorescent lamp isefficiently absorbed by the semiconductor layer. Examples of thepreferred material with such characteristics include metal oxides suchas SnO₂, In₂O₃, ZnO, CdO, CdSnO₄ and ITO (In₂O₃+SnO₂). The upperelectrode (second electrode) can be formed with a known method such asevaporation, sputtering or reactive sputtering.

Current Collecting Electrode

The current collecting electrodes are provided with an intervaltherebetween on the light-receiving side. Further, the currentcollecting electrodes are formed in a comb-like shape on the upper(second) electrode and as an electrode of low resistance for improvingthe energy conversion efficiency of the photovoltaic element, becausethe current collection directly from the second electrode of high sheetresistance results in a low energy conversion efficiency by the highseries resistance. The width and pitch of the current collectingelectrodes are designed so as to minimize electrical resistance incurrent collection and shadow loss. The current collecting electrode isrequired to have a low specific resistivity and not to contribute seriesresistance with respect to the photovoltaic element. The specificresistivity is preferably within a range of 10⁻² to 10⁻⁶ Ωcm. Thecurrent collecting electrode is composed of a metal such as Ti, Cr, Mo,W, Al, Ag, Ni, Cu, Sn, Pt or Cu, an alloy thereof or solder. The currentcollecting electrode can be formed by printing of so-called conductivepaste, which consists of the above-mentioned powdered metallic materialmixed with a polymer binder and a solvent for the binder in anappropriate ratio, plating of the above-mentioned metallic material, orplacing of a wire of the above-mentioned metallic material.

The comb-shaped current collecting electrode may be formed in a desiredshape and position by sputtering, resistance heating or CVD, whileemploying a mask of a desired shape. There may also be employed a methodof evaporating a metal over the entire surface and patterning theobtained metal layer by etching, a method of directly forming thepattern of the current collecting electrode by photo CVD, a method offorming a mask of the negative pattern of the current collectingelectrode followed by plating, or a method of screen printing aconductive paste. The above-mentioned screen printing method consists ofprinting the conductive paste through a screen having a desired patternon a polyester or stainless steel mesh and can provide a currentcollecting electrode of a width of about 50 μm at minimum. The printingcan be advantageously executed in a commercially available screenprinting machine. The screen printed conductive paste is heated in adrying oven, in order to crosslink the binder and to evaporate thesolvent. The drying oven can be a hot air oven or an infrared oven.

The current collecting electrode may also be formed with a metal wire.In such case, there is advantageously employed a metal wire such as Ti,Cr, Mo, W, Al, Ag, Ni, Cu, Sn, Pt or Cu with a diameter preferablywithin a range of 50 μm to 200 μm. The current collecting electrode canbe provided by adhering the metal wire to the second electrode with aconductive adhesive. The metal wire may be coated in advance with theconductive adhesive. Further, the current collecting electrode may beprovided directly on the bypass diode layer. In this case, the upperelectrode must be connected to both the photovoltaic layer and thecurrent collecting electrode.

BusBar

In the present invention, the busbar may be employed if necessary. Thebusbar serves as an electrode for further collecting the currents in thecurrent collecting electrode to an end. The busbar can be composed of ametal such as Ag, Pt or Cu or an alloy thereof. The busbar may be formedin a wire, a foil or a conductive paste similar to that employed for thecurrent collecting electrode. The foil-shaped busbar may be composed,for example, of a copper foil or a tin-plated copper foil, optionallycoated with an adhesive. The busbar may be formed by fixing a metal wirewith conductive adhesive, or by adhering a copper foil. Otherwise it maybe formed in a similar manner to the case of the current collectingelectrode.

Second Semiconductor Junction Layer

The second semiconductor junction layer in the bypass diode layer iscomposed of thin semiconductor layers deposited on the aforementionedsubstrate and having at least a pn or pin junction as described aboveand is formed by using almost the same materials and method as in thecase of the first semiconductor junction layer. The second semiconductorjunction layer may be formed simultaneously with or separately from thefirst semiconductor junction layer. For example, in case thephotovoltaic element is shielded from light, the bypass diode performs afunction of bypassing an operation current generated by otherphotovoltaic elements connected in series and is required to be capableof bypassing the operation current at the operation point of thephotovoltaic element. For this purpose, the operation current and theoperation voltage of the bypass diode are determined according to thedesired specifications of the photovoltaic elements. The operationcurrent of the bypass diode may be increased or decreased, for example,by increasing or decreasing the area of the bypass diode layer or theimpurity density in the semiconductor layer.

In the following there will be explained examples of the presentinvention, but it is to be understood that the present invention is notlimited by these examples.

EXAMPLE 1

FIGS. 3A to 3C are schematic cross-sectional views showing bypass diodeportions in three kinds of photovoltaic elements with bypass diodes,according to the examples of the present invention. As shown in thesedrawings, each of photovoltaic elements 30 is composed of a photovoltaiclayer 32 having a semiconductor junction layer for generating aphotoelectromotive force and a current collecting electrode 34 providedon a bypass diode layer 38. The bypass diode layer 38 is composed of asecond semiconductor junction layer other than the first semiconductorjunction layer in the photovoltaic layer 32.

In the element shown in FIG. 3A, each of the semiconductor junctionlayers of the photovoltaic layer 32 and the bypass diode layer 38 isformed so as to have one pin junction. There is also provided an upperelectrode 33 at the light-receiving surface side. An intrinsic layer 36of the bypass diode layer 38 and an intrinsic layer 35 of thephotovoltaic layer 32 are formed separately. In the element shown inFIG. 3B, each of the semiconductor junction layers of the photovoltaiclayer 32 and the bypass diode layer 38 is formed so as to have one pnjunction formed by thin films, and there is no upper electrode becausethe thin films have a small sheet resistance. In the element shown inFIG. 3C, the intrinsic layer of the photovoltaic layer 32 and theintrinsic layer 37 of the bypass diode layer 38 are commonly provided soas to continuously cover both semiconductor junction layers. FIGS. 4Aand 4B are respectively a plan view seen from the light-receivingsurface side and a cross-sectional view of the entire photovoltaicelement having the configurations shown in FIGS. 3A to 3C. FIG. 4B is across-sectional view along the current collecting electrode 44.

The photovoltaic element shown in FIG. 3A was produced in the followingmanner. At first, a sufficiently degreased and rinsed substrate 31 ofSUS430BA (0.2 mm in thickness) was placed in a RF plasma CVD apparatusnot shown in the drawings and then subjected to deposition of n-type,i-type and p-type layers in this order, thereby obtaining thesemiconductor junction layer of the photovoltaic layer 32. In thisoperation, the substrate 31 was masked with a polyimide adhesive tape inlines of a width of 1 mm spaced with a gap of 5 mm therebetween, inorder to prevent formation of the semiconductor junction layer in themasked area. That is, the semiconductor junction layer of thephotovoltaic layer 32 was formed in plural positions mutually separatedby a gap of 1 mm.

Then the substrate 31 having the semiconductor junction layer formedthereon was placed in a resistance heating type evaporation apparatusnot shown in the drawings and subjected to the evaporation of In-Snalloy by resistance heating under an internal pressure of 1×10⁻⁴ Torrwhile introducing oxygen to deposit a transparent ITO upper electrode 33of a thickness of 700 Å having also an anti-reflective effect, therebycompleting the photovoltaic layer 32.

Then the above-mentioned mask was removed from the substrate 31, thephotovoltaic layer 32 was masked instead, and the above-mentioned CVDfilm forming apparatus was employed to deposit p-type, i-type and n-typelayers in this order to complete the bypass diode 38. The bypass diodelayer 38 having the p-type, i-type and n-type layers was formed so thatthe surface of the bypass diode layer was at the same level as that ofthe photovoltaic layer 32 adjacent to the bypass diode layer. Further,the upper electrode 33 was formed on the bypass diode layer 38 so thatthe surface of the upper electrode 33 formed on the bypass diode layer33 was at the same level as that of the upper electrode formed on thephotovoltaic layer 32.

Subsequently the circumference of the upper electrode 33 was etched toform an area 47 in which the semiconductor was exposed. Then aninsulating tape 45 was adhered to an end of the substrate 31.

Then a copper wire of a diameter of 100 μm, coated with a conductiveresin consisting of carbon black dispersed in urethane resin, waspositioned on the bypass diode layer 38 so as to be in contact with theupper electrode 33 and was fixed onto the upper electrode 33 and theinsulating tape 45 by heating for 10 minutes at 200° C. under a pressureof 1 kg/cm², thereby completing the current collecting electrode 34.

Then a busbar 46 consisting of a copper foil of a thickness of 100 μmwas adhered onto the current collecting electrode 44 so as to overlapthe insulating tape 45, whereby the photovoltaic element with the bypassdiode shown in FIG. 3A was completed.

Ten samples of this photovoltaic element were produced in the samemanner.

Then these samples were subjected to resin-sealing (encapsulation) asfollows. At first, EVA resins were placed on and under the substrate 31.In this case, the bypass diode layer and the photovoltaic layer werecovered with the EVA resin. The EVA resin at the light incident side hada thickness of 250 μm. Further, a fluororesin film was stacked on thelight incident side so as to overlap the EVA resin, while a metal platewas stacked on the back side so as to overlap the EVA resin. Then, thestacked components were subjected to vacuum lamination in a vacuumlaminator for 60 minutes at 150° C. in order to conduct hot-pressing.

Then, each encapsulated sample was subjected to the measurement ofinitial characteristics according to the output measuring method for theamorphous solar cell module, defined in JIS C8935. At first the energyconversion efficiency was determined by measuring the solar cellcharacteristics with a solar simulator light source (manufactured bySPIRE Co., hereinafter referred to as “simulator”) with a light of 100mW/cm² at an AM1.5 global sunlight spectrum. The obtainedcharacteristics were satisfactory with little fluctuation, and theshadow loss was 4.5%.

Then the samples were subjected to reliability measurement according tothe temperature-humidity cycle test A-2 defined in the environmental anddurability test methods for the amorphous solar cell module in JISC8938. Specifically, the sample was placed in a constanttemperature/humidity container with controllable temperature andhumidity and was subjected to a cycle test by varying the temperaturefrom −40° C. to +85° C. (85% relative humidity) ten times. Upon theobservation after testing, the samples showed satisfactory appearancewithout any peeling or bubbling of the laminating materials.

As explained in the foregoing, the photovoltaic element of the presentexample was produced by integrating the bypass diode portion with thephotovoltaic layer and providing the bypass diode layer 38 under thecurrent collecting electrode 34. It is therefore possible to reduceshadow loss, to make planar the entire photovoltaic element, therebyallowing thinner lamination materials to be employed, and to obtainsatisfactory reliability.

COMPARATIVE EXAMPLE 1

For the purpose of comparison, a photovoltaic element 92 with theconventional bypass diode 93 as shown in FIG. 9 was produced insubstantially the same manner as in Example 1. More specifically, thesame method as in Example 1 there was conducted up to the formation ofan upper electrode (not shown in the drawings) on a substrate (not shownin the drawings), and then the current collecting electrode and thebusbar were formed thereon to obtain the photovoltaic element 92.

Subsequently the diode 93 was connected to the photovoltaic element 92,and lamination (encapsulation) was executed in the same manner as inExample 1 to obtain a solar cell module 91. In this operation, thelamination material was varied in thickness as 250 μm, 500 μm, 1 mm and3 mm. The filing was insufficient at thicknesses not larger than 1 mm,but satisfactory at a thickness of 3 mm.

The foregoing results indicate that the photovoltaic element with bypassdiode of Example 1 can provide a solar cell module thinner than thatwith the conventional photovoltaic element 92, thereby allowing for areduction of the amount of lamination material.

EXAMPLE 2

In this example, a photovoltaic element module (not shown in thedrawings) was formed by serial connection of photovoltaic elements 40with bypass diodes having the configuration shown in FIGS. 4A and 4B.Specifically, at first there were produced ten photovoltaic elementswith bypass diodes of the configuration shown in FIGS. 4A and 4B. Thebusbar 46 of each photovoltaic element was connected through aninterconnector to the substrate 41 of an adjacent photovoltaic element,and this connection was repeatedly conducted to obtain ten photovoltaicelements connected in series.

Then the photovoltaic elements connected in series were subjected toencapsulation in the following manner. Specifically, at first, EVAresins were stacked on and under each substrate 41, i.e., on the lightincident side and the opposite side thereof. The EVA resin on the lightincident side had a thickness of 250 μm. Then a fluororesin film wasfurther stacked on the light incident side, while a metal plate wasstacked on the back side. Then the stacked components were subjected tovacuum lamination in a vacuum laminator for 60 minutes at 150° C.

Then, the connected encapsulated photovoltaic elements were subjected tomeasurement of the initial characteristics according to the outputmeasuring method for the amorphous solar cell module, defined in JISC8935. Specifically, at first, the energy conversion efficiency wasdetermined by measuring the solar cell characteristics with the solarsimulator light source (manufactured by SPIRE Co., hereinafter referredto as “simulator”) with light of 10 mW/cm² at an AM1.5 global sunlightspectrum. The obtained characteristics were satisfactory with littlefluctuation.

Then the reliability was measured by the hot spot test A-1 defined inthe environmental and durability test methods for the amorphous solarcell module in JIS C8938. Specifically, at first, the sample wasirradiated with light of 100 mW/cm² in the solar simulator, while onephotovoltaic element among the serially connected ten photovoltaicelements-containing module (ten photovoltaic elements connected inseries) was shadowed. After standing for ten minutes in this condition,the solar cell characteristics were measured with the simulator in thesame manner as in the measurement of the initial characteristics, but nosignificant degradation was observed in comparison with the initialconversion efficiency.

The foregoing results indicate that the solar cell comprising thephotovoltaic elements of the present example has satisfactorycharacteristics, can avoid hot spot damage resulting from a partialshadow, and has high reliability.

COMPARATIVE EXAMPLE 2

For the purpose of comparison, conventional photovoltaic elementswithout the bypass diode were produced in substantially the same manneras in Example 1. More specifically, at first there was executed the samemethod as in Example 1 up to the preparation of the upper electrode onthe substrate. Then an insulating layer with an adhesive material wasadhered to both ends of the substrate, and the current collectingelectrode was formed in the same manner as in Example 1. Then the busbarconsisting of a copper foil was stacked to complete the photovoltaicelement.

Then, ten photovoltaic elements thus produced were connected in series,and the serial photovoltaic elements thus obtained were encapsulated inthe same manner as in Example 1.

The encapsulated photovoltaic elements were subjected to measurement ofthe initial characteristics in the same procedure as in Example 1 and tothe evaluation of the reliability in the same manner as in Example 2.The measurement after tests revealed that the energy conversionefficiency decreased by about 7.5%. The cause of the decrease inefficiency was analyzed and attributed to the shunting of thephotovoltaic element in the shadowed area which was generated by theapplication of a reverse bias to such a shadowed photovoltaic element.

EXAMPLE 3

FIG. 5 is a cross-sectional view of the photovoltaic element of Example3 of the present invention. As shown in FIG. 5, the photovoltaic layer52 in this photovoltaic element 50 has a triple cell structure, in whichthe semiconductor junction layer consists of three stacked semiconductorlayers each including a pin junction. On the other hand, the bypassdiode layer 53 has a single cell structure, in which the semiconductorjunction layer includes only one semiconductor layer having a pinjunction. The photovoltaic layer 52 and the bypass diode layer 53 areformed laterally adjacent to each other on the same substrate 51,wherein the forward directions of the respective semiconductor junctionlayers are mutually opposite to each other. The photovoltaic layer 52and the bypass diode layer 53 are connected to the current collectingdiode 54 at the light incident side. Similarly, there can be formed aphotovoltaic element comprising a photovoltaic layer 52 having a triplecell structure of pn junctions and a bypass diode layer 53 having asingle cell structure of a pn junction.

FIGS. 6A, 6B, 7A and 7B show a method of forming the photovoltaicelement 50 in a roll-to-roll process by a triple cell film-formingapparatus 60. FIG. 6A is a schematic cross-sectional view of thefilm-forming apparatus utilizing the roll-to-roll process. As shown inFIG. 6A, there are arranged film-forming chambers 62 to 70 in thisorder. Among them, the chambers 62, 65 and 68 are used for formingn-type layers, the chambers 63, 66 and 69 are used for forming i-typelayers and the chambers 64, 67 and 70 are used for forming p-type layersto obtain triple cells. FIGS. 7A and 7B are a plan view and across-sectional view showing a state of passing the substrate throughthe chamber provided with a mask (baffle plates). Except for thechambers 64, 66 and 68 for forming the p-type layer of the bottom layer,the i-type layer of the middle layer and the n-type layer of the toplayer, each chamber 72 is provided with baffle plates 73 forintercepting the plasma as shown in FIG. 7B, thereby preventing filmformation on a part of the substrate 51. The baffle plates 73 arepositioned corresponding to the plural current collecting electrodes 54and are provided with a width substantially equal to that of the currentcollecting electrode 54. By employing the above-described structure, thep-type layer of the bottom layer, the i-type layer of the middle layerand the n-type layer of the top layer is formed on a part of thesubstrate 51 to complete the bypass diode layer 53 on the substrate,while forming the semiconductor junction layers of the photovoltaiclayer 52 on the substrate. In other words, the third, fifth and seventhlayers among nine semiconductor layers constituting the semiconductorjunction layer of the photovoltaic layer 52 are respectively formedsimultaneously with the first, second and third layers of threesemiconductor layers constituting the semiconductor junction layer ofthe bypass diode layer 53 by using the same materials.

The above-described apparatus was used for forming the photovoltaicelement 50 shown in FIG. 5 in the following manner. At first, as shownin FIG. 6A, a coiled long SUS430 substrate 51 was set in a feedingchamber 61 of the film-forming apparatus and was transported at aconstant speed to a wind-up chamber 71 at the other end, and thephotovoltaic layer 52 and the bypass diode layer 53 were formed on thesubstrate 51 as shown in FIG. 6B in the course of transportation. Thenanother vacuum chamber not shown in the drawings was used to form theupper electrode only on the photovoltaic layer 52. After the filmformation, the long substrate 51 was cut into sheets of a length of 20cm. Subsequently, an upper electrode at the periphery of each cutsubstrate was removed by etching as in Example 1, and a currentcollecting electrode and a busbar were formed.

Then ten photovoltaic elements 50 thus produced were encapsulated as inExample 1 to obtain ten samples of the photovoltaic element.

The obtained samples were subjected to measurement of the initialcharacteristics in the same manner as in Example 1. The samples showedsatisfactory conversion efficiency within a range of 8.5±1.5%, with ashadow loss as small as 4.5% and with little fluctuation.

EXAMPLE 4

In this example, there was produced a photovoltaic element 80 withbypass diode of a configuration shown in FIG. 8A, and such photovoltaicelement was then divided to obtain separate photovoltaic elements withan arbitrary width such as a photovoltaic element 90 shown in FIG. 8B.

At first, there was executed the process of Example 1 up to theformation of the upper electrode 83, and the upper electrode 83 in aportion to be divided and in the periphery of the substrate to form aregion 87 was exposed. Then the current collecting electrode 84 wasfixed in the same manner as in Example 1. Further, the currentcollecting electrode 84 and the busbar 89 were fixed with a silver pasteto complete the photovoltaic element 80 with a bypass diode of aconfiguration shown in FIG. 8A.

Subsequently, the photovoltaic element 80 thus produced was subjected tomeasurement of the conversion efficiency. Thereafter, the photovoltaicelement was divided into four pieces along the etching lines to obtainthe photovoltaic elements 90 as shown in FIG. 8B. These photovoltaicelements 90 were subjected to measurement of the conversion efficiency.

Comparison of the conversion efficiencies of the photovoltaic elements80 and 90 revealed that the conversion efficiency barely varied beforeand after the division, thereby indicating no damage by division.Consequently it is possible to obtain a photovoltaic element with abypass diode that can be divided into an arbitrary widths.

As explained in the foregoing, the present invention capable ofproviding the bypass diode layer including the second semiconductorjunction under the current collecting electrode by film formation canprovide the photovoltaic element with high conversion efficiency and lowshadow loss without using a complex process.

Also, because the photovoltaic layer and the bypass diode layer areformed in a parallel belt-like shape of substantially the same length,the photovoltaic element can be divided into photovoltaic elements withan arbitrary width.

Furthermore, as the intrinsic layer constituting the first semiconductorjunction layer and the intrinsic layer constituting the secondsemiconductor junction layer are continuously formed, the photovoltaicelement can be produced in a simple manner.

Furthermore, because the first semiconductor junction layer has a triplecell structure consisting of three stacked semiconductor layers eachincluding a pn or pin junction and the second semiconductor junctionlayer has a single cell structure, the photovoltaic element can beproduced in a simple manner.

What is claimed is:
 1. A photovoltaic element comprising: a photovoltaiclayer having a first semiconductor junction layer for generating aphotoelectromotive force, a current collecting electrode provided at alight incident side of the photovoltaic layer, and a bypass diodeconnected in parallel, wherein the bypass diode is formed under thecurrent collecting electrode as a bypass diode layer having a secondsemiconductor junction layer, and wherein each of the first and secondsemiconductor junction layers comprises an intrinsic layer whichcontinuously extends over both semiconductor junction layers and whichextends into the bypass diode layer.
 2. The photovoltaic elementaccording to claim 1, wherein the photovoltaic layer and the bypassdiode layer are formed in a parallel belt-like shape of substantiallythe same length.
 3. The photovoltaic element according to claim 1,wherein the photovoltaic layer has a triple cell structure with thesemiconductor junction layer consisting of three stacked semiconductorlayers each including a pin or pn junction and the bypass diode layerhas a single cell structure with the semiconductor junction layerconsisting of a semiconductor layer including a pin or pn junction. 4.The photovoltaic element according to claim 1, wherein the photovoltaiclayer and the bypass diode layer are formed laterally adjacent to eachother on the same substrate.
 5. The photovoltaic element according toclaim 1, wherein forward directions of semiconductor junctions of thefirst and second semiconductor junction layers are mutually opposite toeach other, and the photovoltaic layer and the bypass diode layer areconnected to the current collecting electrode at the light incidentside.
 6. A method of producing a photovoltaic element according to claim1, wherein the second semiconductor layer is formed so as to be flushwith the first semiconductor layer and the fourth semiconductor layer isformed so as to be flush with the third semiconductor layer.
 7. A methodof producing a photovoltaic element comprising the steps of: forming, ona conductive substrate or a substrate with a conductive film formedthereon, a photovoltaic layer having a first semiconductor junctionlayer for generating a photoelectromotive force in plural positions witha predetermined interval therebetween, forming a bypass diode layerhaving a second semiconductor junction layer with a forward direction ofa semiconductor junction opposite to that of the first semiconductorjunction layer, on the substrate between the plural positions of thephotovoltaic layer, and forming a current collecting electrode on thebypass diode layer so as to be connected with the photovoltaic layer andthe bypass diode layer, wherein each of the first and secondsemiconductor junction layers is formed so as to have a pin junction,and the i-type layer of each of the first and second semiconductorjunction layers is simultaneously formed with the same material andextends into the bypass diode layer.
 8. The method of producing aphotovoltaic element according to claim 7, wherein, at the formation ofthe photovoltaic layer and the bypass diode layer, at least one of thesemiconductor layers constituting the first semiconductor junction layerand at least one of the semiconductor layers constituting the secondsemiconductor junction layer are simultaneously formed with the samematerial.
 9. The method of producing a photovoltaic element according toclaim 8, wherein the first semiconductor junction layer is formed withthree stacked semiconductor layers each including a pin junction and thesecond semiconductor junction layer is formed with a semiconductor layerincluding a pin junction, and wherein a third, a fifth and a seventhsemiconductor layer among nine semiconductor layers constituting thefirst semiconductor junction layer are respectively formedsimultaneously with a first, a second and a third semiconductor layer ofthree semiconductor layers constituting the second semiconductorjunction layer using the same materials.
 10. A photovoltaic elementcomprising: a conductive plane; a photovoltaic layer and a bypass diodelayer provided on the conductive plane; and a current collectingelectrode provided on the bypass diode layer; wherein a plurality ofseparate photovoltaic layers is provided on the conductive plane, thebypass diode layer is provided in a separation region between adjacentphotovoltaic layers and in contact with the plurality of photovoltaiclayers.
 11. The photovoltaic element according to claim 10, wherein thephotovoltaic layer has a first semiconductor junction layer, wherein thebypass diode layer has a second semiconductor junction layer, andwherein a forward direction of the first semiconductor junction layerand a forward direction of the second semiconductor junction layer areopposite with respect to a normal direction of the conductive plane, andthe first semiconductor junction layer and the second semiconductorjunction layer are in contact with each other.
 12. The photovoltaicelement according to claim 10, wherein the photovoltaic layer and thebypass diode layer are flush with each other.
 13. The photovoltaicelement according to claim 10, wherein the photovoltaic layer has anintrinsic layer and the intrinsic layer extends into the bypass diodelayer.
 14. A method of producing a photovoltaic element comprising thesteps of: forming a plurality of photovoltaic layers provided with aseparation region on a conductive plane; forming a bypass diode layer inthe separation region, wherein the bypass diode layer is formed so thatthe bypass diode layer is brought into contact with the plurality ofphotovoltaic layers; and forming a current collecting electrode on thebypass diode layer.
 15. The method of producing a photovoltaic elementaccording to claim 14, wherein the photovoltaic layer has a firstsemiconductor junction layer, wherein the bypass diode layer has asecond semiconductor junction layer, and wherein a forward direction ofthe first semiconductor junction layer and a forward direction of thesecond semiconductor junction layer are opposite with respect to anormal direction of the conductive plane, and the first semiconductorjunction layer and the second semiconductor junction layer are incontact with each other.
 16. The method of producing a photovoltaicelement according to claim 14, wherein the bypass diode layer is formedso as to be flush with the photovoltaic layers.
 17. A method ofproducing a photovoltaic element comprising the steps of: forming afirst semiconductor layer constituting a part of a plurality ofphotovoltaic layers provided with a separation region on a conductiveplane; forming a second semiconductor layer constituting part of abypass diode layer in the separation region; forming an intrinsic layeron the first and the second semiconductor layers; forming a thirdsemiconductor layer constituting a part of the plurality of thephotovoltaic layers on the first semiconductor layer through theintrinsic layer; forming a fourth semiconductor layer constituting apart of the bypass diode layer on the second semiconductor layer throughthe intrinsic layer; and forming a current collecting electrode on thefourth semiconductor layer.
 18. The method of producing a photovoltaicelement according to claim 17, wherein the photovoltaic layer has afirst semiconductor junction layer having the first and the thirdsemiconductor layers, wherein the bypass diode layer has a secondsemiconductor junction layer having the second and the fourthsemiconductor layers, and wherein a forward direction of the firstsemiconductor junction layer and a forward direction of the secondsemiconductor junction layer are opposite with respect to a normaldirection of the conductive plane, and the first semiconductor junctionlayer and the second semiconductor junction layer are in contact witheach other.
 19. The method of producing a photovoltaic element accordingto claim 17, wherein the second semiconductor layer is formed so as tobe flush with the first semiconductor layer.
 20. A method of producing aphotovoltaic element according to claim 17, wherein the fourthsemiconductor layer is formed so as to be flush with the thirdsemiconductor layer.